EP3543682B1 - Method of operating an optical measuring system for measuring the concentration of a gas component in a gas to be measured - Google Patents

Description

The invention relates to a method for operating an optical measuring system for measuring the concentration of a gas component in a measuring gas, based on the wavelength modulation spectroscopy, with a wavelength-tunable temperature-stabilized laser light source which has a central base wavelength λ _{0 of} the laser light of the laser light source periodically by changing the basic current over a Absorption line of interest of the gas component varies at an operating point and at the same time modulates with a frequency (f) and a determinable amplitude by means of a modulation device, a light detector which detects the intensity of the laser light after passing through the measurement gas, and with an evaluation device, the means for phase-sensitive Contains demodulation of a measurement signal generated by the light detector at frequency (f) and / or one of its harmonics, the laser light source having a base current I _DC and a modulation trom I _AC is operated with current modulation and a laser beam of wavelength λ _{0 is} emitted with a wavelength modulation amplitude Δλ _AC and the wavelength modulation amplitude Δλ _{AC of} the laser light is kept constant by means of a variable setting of the current modulation amplitude ΔI _AC .

Optical measuring systems for measuring the concentration of a gas component in a measuring gas, based on wavelength modulation spectroscopy, are known from the prior art in a variety of embodiments, as are a multitude of different methods for operating such an optical measuring system.

In the tunable laser absorption spectroscopy (TLAS), especially the wavelength modulation spectroscopy (WMS), the wavelength modulation amplitude generally plays an important role. This is defined by setting the current modulation amplitude during calibration for the operating point. Changes in the operating point or long-term changes in the change in the wavelength modulation amplitude also cause the sensor calibration to run out of the specification limits, which then often necessitates a recalibration of the optical measuring system.

From the DE 41 10 095 A1 A method for gas spectroscopic measurement is known with a laser diode operated with a modeled control current, a monitor diode, a detector device for receiving a measurement signal of the transmitted radiation and a lock-in amplifier, in which the offset component in the output signal of the lock-in amplifier is eliminated shall be. This is achieved in that the optical radiation power of the laser diode is regulated to a predetermined modulation profile with the monitor diode as the actual value transmitter. There, the optical output power profile is kept constant, which does not mean, however, that the same wavelength modulation profile always automatically results from this. Constant optical power modulation does not automatically result in constant wavelength modulation.

To control a wavelength-tunable laser diode in a spectrometer, according to DE 10 2013 202 289 A1 Instead of a current-time function, a power-time function is specified, according to which the laser diode is periodically tuned over a wavelength range. For this purpose, the power-time function and measured values on the laser diode applied voltage determines a current curve with which the laser diode is controlled. The electrical power is measured here, but it is the total laser power including the voltage across the ideal diode. This method is too imprecise in certain applications.

The EP 2 848 918 A1 describes a gas analyzer for measuring the concentration of a gas component in a measurement gas based on the method of wavelength modulation spectroscopy. The special feature is that a reference detector or a measuring detector is controlled as a function of its demolished output signal, the modulation intensity is controlled and there is a computing device which generates a model signal for the demolished measuring signal or reference signal, forms a difference signal between the model signal and the demodulated measuring signal or reference signal and controls the modulation intensity with the difference signal. This document therefore describes a system for continuous self-calibration and zero point correction, a wavelength modulation stroke not being regulated.

It is known in the case of generic optical measuring systems, but in the case of lasers this leads to the wavelength modulation amplitude changing (despite the unchanged current modulation amplitude). As a result

Changes in the operating point or in the event of long-term changes in the laser light source compared to the calibration time to stabilize the sensor accuracy, to reset the wavelength modulation amplitude during operation by adjusting the intensity of the modulation current for the laser light source, that is to say the current modulation amplitude in such a way that the wavelengths -Modulation amplitude again corresponds at least approximately to the time of calibration. In this regard, an example is given in the publication EP 2 610 608 B1 referred.

The font EP 2 610 608 B1 discloses a gas measuring device for measuring a target gas and a method for adjusting a width of a Wavelength modulation of the gas measuring device with a light source and a detection unit by oscillating a wavelength of a laser light from the source to have a central wavelength determined by a main current and according to a modulation current at an oscillation frequency and with a width of a wavelength modulation while the central wavelength is varied by changing the main current with a longer cycle than that of the modulation current, whereby the detection unit outputs a signal according to an intensity of the laser light transmitted by a standard gas. The method further includes obtaining a detection signal by detecting the laser light transmitted through the standard sample while varying the central wavelength, obtaining a specific frequency component of the detection signal that oscillates at a frequency that is a positive integer multiple of an oscillation frequency of the modulation current and calculating a ratio of a size of a local minimum of the specific frequency component with respect to the central wavelength of the laser light and a size of a local maximum of the specific frequency component with respect to the central wavelength of the laser light, and adjusting the width of the wavelength modulation of the laser light so that the Ratio meets a predetermined condition. This is a condition in which the ratio corresponding to the width of the wavelength modulation one to one equals a predetermined target value. When adjusting the modulation width of the laser light, the width of the wavelength modulation is adjusted by adjusting an intensity of the modulation current.

Proceeding from this, the object of the claimed invention is to propose another simpler and more precise possibility of changing the wavelength modulation amplitude despite changing laser properties, e.g. Temperature, operating current, long-term drift, to keep constant.

This object is achieved according to the invention by a method for operating an optical measuring system for measuring the concentration of a gas component in a measuring gas, based on the wavelength modulation spectroscopy, with the features of independent claim 1. Further advantageous embodiments can be found in the related claims.

In optical measuring systems for measuring the concentration of a gas component in a measuring gas, based on the wavelength modulation spectroscopy, the wavelength modulation amplitude Δλ _{AC is} the decisive parameter that is used during the calibration of the sensor via the current modulation amplitude ΔI _AC for the selected operating point I _{DC is} set. However, deviations from the operating point, for example due to a change in the outside temperature relative to the calibration temperature or due to wavelength drift due to aging of the laser, lead to a change in the wavelength modulation amplitude. As a result, the sensor accuracy is reduced and recalibration may be required.

The invention is based on the main idea that, when the measuring system is operated at the intended operating point, the wavelength modulation amplitude Δλ _{AC of} the laser light using operating parameters set and then recorded and stored for the operating point at the time of calibration of the laser light source, preferably a laser diode, and during operation stabilize measured voltages and / or currents at the laser light source. During the development of the new method, it was found that the wavelength modulation amplitude Δλ _{AC is} proportional to the modulated AC power ΔP _AC . $${\mathrm{\Delta P}}_{\mathrm{AC}}\sim {\mathrm{\Delta \lambda }}_{\mathrm{AC}}$$

According to the invention, the wavelength modulation amplitude Δλ _{AC of} the laser light is kept constant via a variable setting of the current modulation amplitude ΔI _AC by keeping a modulated power ΔP _{AC of} the laser light at an internal resistance R _{I of} the laser light source constant at the operating point. Among other things, the voltage at the laser light source is measured.

It is generally known that each laser light source circuit diagram can be replaced by an equivalent circuit diagram which comprises a laser emitter (active zone) and an internal resistance R _I connected in series therewith. As soon as a basic current I _DC modulated with a modulation current I _{AC is} generated by the laser light source flows, there is a voltage U _L at the laser light source, which drops in part at the laser emitter as partial voltage U _E and at the internal resistance Ri as partial voltage U _Ri .

The voltage across the laser U _L is not relevant for the calculation of the modulated power, but only the voltage U _Ri dropped across the internal resistance R _I. The power modulation amplitude ΔP _{AC is} calculated as follows: $${\mathrm{\Delta P}}_{\mathrm{AC}}={\mathrm{\Delta I}}_{\mathrm{AC}}\cdot {\mathrm{U}}_{\mathrm{Ri}}={\mathrm{\Delta I}}_{\mathrm{AC}}\cdot {\mathrm{R}}_{\mathrm{I.}}\cdot {\mathrm{I.}}_{\mathrm{DC}}$$

Where ΔI _AC denotes the current modulation amplitude and U _{Ri represents} the voltage dropped across the internal resistance. Alternatively, the voltage can also be calculated from the value of the internal resistance R _I and the DC laser current I _DC flowing through it.

wobei U_E je nach Lasertyp (mit Telekom-naher Wellenlänge) einen Wert von 0.9-1.1 V aufweist.The DC voltage across the internal resistance Ri results in $${\mathrm{U}}_{\mathrm{Ri}}={\mathrm{U}}_{\mathrm{L}}-{\mathrm{U}}_{\mathrm{E}}$$

where U _{E has} a value of 0.9-1.1 V depending on the laser type (with a wavelength close to Telekom).

In general, it should be noted that when the optical measuring system is operated in a non-thermostabilized environment, a change in the ambient temperature leads to the laser light source of the sensor adopting a slightly higher or lower temperature. Thus the target wavelength of the sensor shifts to a lower or higher DC laser current I _DC . As a result, the power modulation amplitude .DELTA.P _AC also changes in accordance with Formula 2. As a consequence of this change in power modulation amplitude, the wavelength modulation amplitude .DELTA.λ _AC also changes in accordance with Formula 1.

Even with a constant DC laser current I _DC , the power modulation amplitude ΔP _{AC can} change if the internal resistance R _{I of} the laser light source changes due to various (long-term) influences during operation.

In order to ensure that the laser light _source is operated at the same time with the same wavelength modulation amplitude Δλ _AC , the current power modulation amplitude ΔP _{AC_Act is kept} equal to the power modulation amplitude ΔP _{AC_Calib} at the time of the calibration of the optical measuring system. $${\mathrm{\Delta P}}_{\mathrm{AC}\_\mathrm{Act}}={\mathrm{\Delta P}}_{\mathrm{AC\_Calib}}$$

wobei die kalibrierten Werte gegenüber den aktuellen Spannungs-Werten ins Verhältnis gesetzt werden. Die Formel 5 ist noch weiter auflösbar. Die Spannung U_Ri am Innenwiderstand Ri kann nach der Formel 3 als U_L - U_E dargestellt werden.If you use Formula 2 in Formula 4 and then solve for the current current modulation amplitude ΔI _{AC_Act} , the result is $${\mathrm{\Delta I}}_{\mathrm{AC}\_\mathrm{Act}}={\mathrm{\Delta I}}_{\mathrm{AC}\_\mathrm{Calib}}\cdot {\mathrm{U}}_{\mathrm{Ri}\_\mathrm{Calib}}/{\mathrm{U}}_{\mathrm{Ri}\_\mathrm{Act}}$$

whereby the calibrated values are compared to the current voltage values. Formula 5 is even more resolvable. The voltage U _Ri at the internal resistance Ri can be represented according to formula 3 as U _L - U _E.

In a preferred embodiment of the method according to the invention, the current modulation amplitude .DELTA.I _AC is adjusted accordingly, taking into account a current modulation amplitude .DELTA.I _{AC_Calib} set when the laser light source is _{calibrated, in} such a way that the modulated power modulation amplitude _{.DELTA.P AC is} kept constant.

In an advantageous embodiment of the invention, in order to stabilize the wavelength modulation amplitude Δλ _AC, the voltage change ΔU _Ri at the internal resistance R _{I of} the laser light source is determined by adapting the current modulation amplitude ΔI _AC , the adaptation of the current modulation amplitude ΔI _{AC being} the quotient of U _{L_Calib} and U _{L_Act} is used.

In a particularly favorable variant of the method according to the invention, the internal resistance Ri of the laser light source is advantageously determined from a voltage / current characteristic curve of the laser light source, in which the voltage drop U _L at the laser light source is recorded as a function of the base current I _DC . The voltage / current characteristic of the laser light source is Usually recorded for the first time during the calibration of the optical measuring system. This can be repeated later if necessary, for example when calibrating again or for checking during normal operation.

In one embodiment of the invention, at least the modulated power ΔP _{AC of} the laser light source is determined during the calibration of the optical measuring system. Instead of the power ΔP _AC , the amplitude ΔI _{AC of} the modulation current I _AC and the voltage drop U _I at the internal resistance R _I can also be used. Alternatively, the amplitude ΔI _{AC of} the modulation current I _AC , the internal resistance R _I and the base current I _DC can be determined at the operating point. The determined parameters are saved at the time of calibration. The base current I _DC determines the operating point of the optical measuring system. This enables deviations in this regard to be recognized in a simple manner during operation and to be reacted to in accordance with the method described above.

The optical measuring system used to carry out the method according to the invention for measuring the concentration of a gas component in a measuring gas, based on the wavelength modulation spectroscopy, has a wavelength-tunable temperature-stabilized laser light source, preferably a semiconductor laser light source in the form of a laser diode, which has a central basic wavelength λ _{0 of} the laser light from the laser light source periodically varied over an absorption line of interest of the gas component at a working point, for example ramp-like (sawtooth), and simultaneously modulated with a frequency (f) and an amplitude by means of a modulation device. The measuring system also has a light detector which detects the intensity of the laser light after it has passed through the measurement gas, and an evaluation device which contains means for phase-sensitive demodulation of a measurement signal generated by the light detector at frequency (f) and / or one of its harmonics . The modulation can be sinusoidal or triangular, for example. The laser light source is operated current modulated with a basic current I _DC and a modulation current I _AC and emits a laser beam of wavelength λ ₀ with a wavelength modulation amplitude Δλ _AC . The wavelength modulation amplitude Δλ _{AC of} the laser light is kept constant by the evaluation device in conjunction with the modulation device via a variable setting of the current modulation amplitude ΔI _AC .

In the calibration and in the regular operation of the optical measuring system for determining the concentration of a gas component, an evaluation device with lock-in technology is preferably used in order to achieve noise reduction in a known manner, in particular to clearly show the noise caused by the 1 / f signal to mitigate. The structure and mode of operation of a lock-in amplifier are generally familiar to the person skilled in the art, so that a comprehensive description is not necessary. In brief, a lock-in amplifier, sometimes referred to as a phase sensitive rectifier or carrier frequency amplifier, is an amplifier for measuring a weak alternating electrical signal that is modulated with a reference signal known in frequency and phase. The device is an extremely narrow-band bandpass filter and thus improves the signal-to-noise ratio. The advantage of using such a device is that direct voltages and alternating voltages of different frequency and noise are efficiently filtered.

The method according to the invention requires an electrical line from the laser light source to the evaluation unit in order to determine the power modulation amplitude ΔP _{AC of} the laser light as precisely as possible via the internal resistance Ri of the laser light source. If this is not available in an optical measuring system provided for using the proposed new method, hardware adaptation is necessary, otherwise this method cannot be used. The construction of an optical measuring system that is suitable for determining the voltage at the internal resistance of the laser light source via a voltage measurement on the laser light source and then adjusting the current modulation amplitude ΔI _{AC_Act} to stabilize the wavelength modulation amplitude Δλ _{AC of} the laser light explained briefly again below using a schematic representation.

The optical measuring system is largely calibrated using the usual method known to those skilled in the art, using a known reference gas which is to be detected as a measuring gas in normal operation by the optical measuring system and whose concentration is to be measured. First, to determine the operating point, the laser light source is operated with a normal base current I _DC and modulation current I _AC and the temperature of the temperature-stabilized laser light source is changed until an absorption signal for the reference gas is detected. The base current I _{DC is then} modulated with a modulation current I _AC at the determined temperature, which is stabilized, for example, by means of a Peltier element, so that the laser light source emits a laser beam of wavelength λ ₀ with a wavelength modulation amplitude Δλ _AC . The current modulation amplitude or the wavelength modulation amplitude can then be optimized in accordance with various criteria of the measuring system, for example an optimal signal-to-noise ratio. This is particularly necessary because the wavelength modulation amplitude Δλ _{AC changes} with the base current I _DC . The wavelength modulation amplitude of the laser light increases with increasing base current, ie it is smaller below a selected working point than above the working point and must therefore be defined separately for each working point.

This basic procedure also includes taking into account measures in wavelength modulation spectroscopy that are suitable for completely or partially suppressing optical interference phenomena. In this way, the wavelength modulation amplitude or the current modulation amplitude is fundamentally determined based on various criteria of the measuring system and the gas to be detected. The corresponding modulated power, ie the power modulation amplitude ΔP _AC, or an equivalent variable, such as the modulation current and the voltage the laser light source is also determined and stored at the time of calibration.

The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the combination indicated in each case, but also in other combinations or on their own.

Figur 1

ein für die Durchführung des erfindungsgemäßen Verfahrens geeignetes optisches Messsystem;

Figur 2

das Ersatzschaltbild für die Laserlichtquelle;

Figur 3

eine aufgenommene Spannungs-Strom-Kennlinie zur Bestimmung des Innenwiderstandes der Laserlichtquelle; und

Figur 4

ein Ablaufschema für die Anpassung der Strom-Modulationsamplitude.

The invention is explained in more detail below with reference to the accompanying drawings. In a schematic representation:

Figure 1

an optical measuring system suitable for carrying out the method according to the invention;

Figure 2

the equivalent circuit diagram for the laser light source;

Figure 3

a recorded voltage-current characteristic curve for determining the internal resistance of the laser light source; and

Figure 4

a flow chart for the adjustment of the current modulation amplitude.

The Figure 1 shows schematically the basic structure of an optical measuring system 1 for measuring the concentration of a gas component in a measuring gas 2, based on the wavelength modulation spectroscopy. The measuring system 1 has a wavelength-adjustable temperature-stabilized laser light source 3, a modulation device 4, a light detector 5, and an evaluation device 6. The laser light source 3 emits a laser beam 7 of wavelength λ ₀ with a wavelength modulation amplitude Δλ _AC . The The modulation device 4 varies the central base wavelength λ _{0 of} the laser light from the laser light source 3 periodically over an absorption line of interest of the gas component at a working point and, at the same time, modulates it triangularly with a frequency (f) and an amplitude. It also includes at least one DC and / or AC voltage source or a DC and AC current source 4a and associated modulation means 4b for operating the laser light source 3. The modulation device 4 is connected directly to the laser light source 3. The light detector 5 detects the laser beam 7 emanating from the laser light source 3 after it has passed the measurement gas 2 and generates a reception signal which is dependent on the intensity of the laser light after passing through the measurement gas 2 and is supplied to the evaluation unit 6. The evaluation unit 6 comprises means for phase-sensitive demodulation of a measurement signal generated by the light detector 5 at the frequency (f) and / or one of its harmonics. The evaluation unit 6 has two lock-in amplifiers 6a, 6b and a computing unit 6c. The computing unit 6c evaluates the demodulated received signal of the light detector 5 and, depending on this, controls the modulation means 4b of the modulation device 4 in order to keep the wavelength modulation amplitude Δλ _{AC of} the laser light constant by adapting the current modulation amplitude ΔI _AC . For this purpose, it has an electrical control line 8 to the modulation device 4. Furthermore, an electrical connection line 9 leads from the laser light source 3 to the lock-in amplifier 6b, with which the voltage applied to the laser light source 3 is detected and evaluated. The formulas 1 to 5 listed above are used in the evaluation.

The Figure 2 represents the equivalent circuit diagram for the laser light source 3. The laser light source 3 can accordingly be replaced by a light emitter 3a and an internal resistance R _I , 3b arranged in series therewith. The laser light source 3 is operated current modulated with a base current I _DC and a modulation current I _AC . Voltage U _{L is present} at laser light source 3, which drops in part at internal resistance 3b as partial voltage U _Ri and at light emitter 3a as partial voltage U _E.

The Figure 3 illustrates a current / voltage characteristic curve 10 recorded during the calibration of the optical measuring system 1 for determining the internal resistance R _{I of} the laser light source 3. The internal resistance Ri is determined from the relationship of the current-voltage characteristic of the laser light source 3 at the operating point 12. For this purpose, the current-voltage characteristic curve 10 (solid line) is provided with a linear approximation line 11 (dashed line) at the operating point 12 for determining the internal resistance Ri. The slope of the approximation line 11 corresponds to the internal resistance R _I , 3b at the working point 12.

The Figure 4 illustrates the flow diagram for the adaptation of the current modulation values, the determination of the currently modulated power either being carried out with the DC laser current which is assigned to the maximum of the absorption signal, or being determined via several DC laser current values of the scan of the measurement. As a rule, several scans are carried out over the absorption line of interest, ie the corresponding central base wavelength λ _{0 of} the laser light from the laser light source is varied several times periodically over the absorption line of the gas component at the working point 12 and modulated at the same time, a number of measuring points usually being recorded and evaluated mathematically . The current modulation amplitude can be maintained for each measuring point, or can be calculated from the calculation of the current measuring point to optimize the result for the next measuring point. In the in the Figure 4 The method sequence shown symbolically, which reflects the current operation of the optical measuring system, is used to determine the current modulated AC power at the internal resistance R _I in a first method step S1. The current current modulation value is then determined in the subsequent second method step S2. In the next subsequent third method step S3, the actual concentration measurement for the sample gas is then carried out with the current current modulation value. The process steps S1 to S3 are carried out several times in a loop, the current modulation amplitude ΔI _AC between the process runs if required can be adjusted if the current current modulation value _ΔI AC_Act _deviates from the ideal current modulation value ΔI _{AC_Calib} during the calibration of the optical measuring system and thus the wavelength modulation amplitude Δλ _{AC is changed} compared to the calibration.

Claims (7)

1. A method for operating an optical measuring system (1) for measuring the concentration of a gas component in a measured gas (2), based on wavelength modulation spectroscopy, comprising a wavelength-tunable temperature-stabilized laser light source (3), which periodically varies a central base wavelength λ₀ of the laser light of the laser light source (3) about a relevant absorption line of the gas component at an operating point (12) and, at the same time, modulates the same with a frequency (f) and an amplitude, by way of a modulation device (4), a light detector (5), which detects the intensity of the laser light after it has passed through the measured gas (2), and an evaluation device (6), which comprises means (6a) for the phase-sensitive demodulation of a measuring signal generated by the light detector (5) at the frequency (f) and/or one of the harmonics thereof, the laser light source (3) being operated in a current-modulated manner with a base current I_DC and a modulation current I_AC and emitting a laser beam (7) of the wavelength λ₀ having a wavelength modulation amplitude Δλ_AC, and the wavelength modulation amplitude Δλ_AC of the laser light being kept constant by way of variable setting of the current modulation amplitude ΔI_AC, characterized in that the voltage at the laser light source (3) is measured at the operating point (12), and based thereon a modulated power ΔP_AC at an internal resistor R_I (3b) of the laser light source (3) is kept constant.
2. The method according to claim 1, characterized in that the current modulation amplitude ΔI_AC is adapted, taking a current modulation amplitude ΔI_{AC_Calib} that was set during the calibration of the laser light source into consideration, such that the modulated power modulation amplitude ΔP_AC is kept constant.
3. The method according to claim 1 or 2, characterized in that the voltage change ΔU_Ri at the internal resistor R_I (3b) of the laser light source (3) is determined by adapting the current modulation amplitude ΔI_AC so as to stabilize the wavelength modulation amplitude Δλ_AC.
4. The method according to claim 3, characterized in that the adaptation of the current modulation amplitude ΔI_AC is based on the quotient of U_{L_Calib} and U_{L_Act}.
5. A method according to any one of the preceding claims, characterized in that, during the calibration of the optical measuring system (1), the modulated power ΔP_AC of the laser light source (3), the amplitude ΔI_AC of the modulation current I_AC and the voltage drop U_I across the internal resistor R_I (3b) or the amplitude ΔI_AC of the modulation current I_AC, the internal resistance R_I and the base current I_DC are ascertained at the operating point (12), and stored.
6. A method according to any one of the preceding claims, characterized in that the internal resistance R_I (3b) of the laser light source (3) is determined from a voltage/current characteristic curve of the laser light source (3) in which the voltage drop U_L across the laser light source (3) is recorded as a function of the base current I_DC.
7. A method according to any one of the preceding claims, characterized in that an evaluation device (6) comprising lock-in technology is used.

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